The present invention pertains to impedance transformation networks typically employed with RF circuitry of a television receiver.
Impedance transformation networks are utilized in RF circuitry to step up or step down the impedance level between pairs of ports of input and output circuits with different source and load impedances, respectively. Most often the impedance transformation network is arranged so that the load impedance provided to the input circuit by the impedance transformation network matches as nearly as possible the source impedance of the input circuit and the source impedance provided to the output circuit by the impedance transformation network matches as nearly as possible the load impedance of the output circuit. Impedance matching is desirable since under these conditions there is a maximum power transfer and a minimum of signal distortion between the input and output circuits.
For these reasons, an impedance transformation network is generally coupled between a UHF television antenna network typically having an impedance of 75 ohms and UHF RF signal processing circuitry included in the receiver typically having an impedance of approximately 300 ohms. In such an arrangement, the impedance transformation network not only matches the impedance levels of the antenna network and RF circuitry as nearly as it can, but is additionally arranged to provide transformation between a balanced impedance configuration of the antenna network to the unbalanced impedance configuration of the RF circuitry of the recevier. A balanced impedance configuration is one in which the impedances between each of two points of a network to signal ground are equal. An unbalanced impedance configuration is one in which the impedances between each of two points of a network to signal ground are not equal. This occurs most frequently when one of the two points is connected to signal ground. An impedance transformation network for coupling a balanced impedance network to an unbalanced impedance network is generally called a balun.
When impedances are not matched distortion may arise due to the establishment of signal reflections at the ports of both the input and output circuits. A factor of merit referred to as the voltage standing wave ratio or VSWR indicates the amount of impedance mismatch between an input and an output circuit and is accordingly indicative of the amount of signal distortion between the input and output circuit. Therefore, it is desirable that an impedance transforming network provide the lowest possible VSWR (unity being the theoretical minimum VSWR).
Impedance transformation networks with a low VSWR are not only desirable for use between an antenna network and the RF circuitry of a television receiver but are also desired for use in an arrangement for testing the noise performance of portions of the receiver such as the RF circuitry. In such a testing arrangement, a noise signal of known amplitude is applied to the antenna input of a receiver through an impedance transformation network and the noise signal amplitude is measured at a desired point in the RF circuitry. A comparison of the noise signal amplitudes at the antenna input and the measurement point provides an indication of the noise contributed or rejected by the RF circuitry.
It has been found that the accuracy of noise measurement is adversely affected by impedance mismatches between the noise source and antenna input. In more precise terms, the accuracy of the measurement is rapidly degraded with increases in the VSWR of the impedance transformation network. Accordingly, impedance transformation networks which provide a desired impedance transformation ratio (e.g., one to four) substantially uniformly throughout the frequency range of interest (e.g., the UHF range approximately 300 MHz to 900 MHz) are desired.
A variety of impedance transformation networks are known. Those employing a simple transformer with single conductor primary and secondary windings about a ferrite core tend to have poor high frequency response characteristics since they exhibit less magnetic coupling and more stray parasitic reactances as the frequency increases. Those employing simple transmission lines tend to have limited bandwidths since their low frequency response is adversely effected by the formation of undesired parasitic transmission line structures to structural members at signal ground potential.
Impedance transformation networks having a hybrid structure combining transmission line with a ferrite core have been employed in high frequency-broadband applications since they obviate the deficiencies of the impedance transformation individual simple structures. Typically they include two separate transmission lines each having two conductors uniformly spaced from one another in what is often referred to as a bifilar wire configuration. Each bifilar wire is wound around a ferrite core and often passes through a respective aperture in the ferrite core. The ends of the conductors of the two bifilar wires are connected so as to provide the desired impedance transformation.
Since such hybrid impedance transformation networks typically require two bifilar wires wound around a ferrite, they tend to be relatively difficult to manufacture. In addition, it has been found that many hybrid impedance transformation networks including bifilar windings do not provide a sufficiently low VSWR over the entire frequency range of interest for accurate noise measurements.